When a non-ferrous electroconductive plate is rotated between two fixed discs containing permanent magnets ("magnetic disc") arranged so that opposing magnets on the discs are of opposite polarity, eddy currents are generated in the rotating plate resulting in magnetic friction between the electroconductive plate and the magnetic discs. Such an arrangement incorporated as a resistance applying means on an exercise bike is disclosed in U.S. Pat. No. 4,826,150. The amount of the drag resulting from the magnetic friction in such a device may be varied by adjusting the relative positions of the magnetic discs between a position in which magnets of opposite polarity are positioned directly opposite one another (maximum magnetic friction) to a position in which magnets of like polarity are positioned directly opposite one another (no magnetic friction). Magnetic friction can also be varied by adjusting the air gaps between the electroconductive plate and the magnetic discs; increasing the gaps decreases the magnetic friction.
It is to be understood that the operation of a load applying device in which a non-ferrous electroconductive plate (copper plate, for example) is rotated relative to an adjacent magnetic disc, is different from the operation of a magnetic coupling device in which a ferrous plate is rotated relative to an adjacent magnetic disc in that in the latter instance there is a relatively strong axial attraction between the ferrous plate and the magnetic disc which is not present in the other instance between the non-ferrous electroconductive plate and the magnetic disc. It has been found that when a copper plate is rotated relative to a co-axial adjacent magnetic disc which is free to rotate and move axially, the magnetic disc will repel and rotate with the copper plate, moving toward the copper plate axially as the rotational speed builds up, but will not normally contact the copper plate. The axial thrust developed between the copper plate and the magnetic disc is proportional to their speed difference. However, when the adjacent rotating plate is ferrous rather than copper, the magnetic plate will move directly into contact with the magnetic plate while stationary or rotating if permitted to do so. This operating distinction is significant in the operation of the present invention.
When a magnetic disc is free to rotate between and independently of a pair of adjacent non-ferrous electroconductive plates which are mounted for rotation on a rotary axis coaxial with the rotary axis of the magnetic disc, and the magnetic disc is driven, for example, relative to the electroconductive plates, the plates initially tend to axially repel away from the magnetic disc as the plates' rotational speed increases and the slip between them decreases. The axial repulsion will then decrease, and the copper plates will eventually move axially toward the magnetic disc, ordinarily maintaining a small air gap which is usually at least about 3 mm. This is not true when ferrous plates are used instead of non-ferrous electroconductive plates adjacent to the magnetic disc.
In all instances herebefore known to applicant in which a non-ferrous electroconductive plate has been used in association with a magnetic disc for a coupling function, the plate has either been positioned between two magnetic discs as in the previously mentioned U.S. Pat. No. 4,826,150, or has been placed between a disc containing a permanent magnet and a yoke element engaging the disc so as to be magnetized. The latter arrangement is utilized in the speed governor disclosed in U.S. Pat. No. 4,826,150.
To applicant's knowledge the prior art has failed to recognize the advantages to be gained in magnetic couplers by arranging a magnetic disc between two adjacent non-ferrous electroconductive plates. The present invention aims to provide improved couplers incorporating this superior arrangement.